Help, uninstaling plugin

Hi All...I'm a newbie when come to Omeka, just install and learn how to use it a 1 week ago.
I try to add custom 'fields' on the existing DC, so I download DCExtended and modified the elemets.php. At first I just tried 2 new fileds, it works, my fields appeared on the add new Collection page.

I thought I can edit more fields, but now, my new fields did not appeared and my previous 2 fields were gone as well.

So I tried to uninstall it, the page went blank.
I tried deactivate it and uninstall, again it went blank.

Any help would be greatly appreciated.

Thank you so much.